Geophysical investigations of the crust and mantle beneath Texas’ Gulf Coastal Plain

Abstract:

EarthScope’s Transportable Array (TA) and a dense broadband profile (X4) that coincided with TA deployments in Texas formed the basis for higher resolution geophysical investigations than previously possible in the Gulf Coast Plain (GCP). X4 is a 300-km-long array of 26 broadband seismic stations deployed from Matagorda Island across the GCP, ending near the center of the Llano uplift (LU). The study found an 80-100 km wide “lithosphere-asthenosphere transition zone” (LATZ). While a negative Sp phase at ~110 km depth could be interpreted as a shallow LAB, several negative Sp phases were also found in the 110-180 km depth interval and this set of negative Sp phases is bounded by a positive Sp phase at 180-200 km depth that is semi-continuous across the length of the array. We interpret the depth interval’s negative anomalies to be the results of partial melt and shearing in a softened or underdeveloped lithosphere, thus the term LATZ. The GCP was also found to have large SKS splitting delay times, which we believe is due to seismic anisotropy in the LATZ (the zone of assumed shearing). Body wave tomography found slightly high Vp but unusually low Vs beneath the GCP; Vs is more sensitive to partial melt and anisotropy than Vp, further supporting the LATZ hypothesis.

Ps and Sp Moho phases are observed at ~35 km beneath the LU but both Sp and Ps RFs fail to image a Moho beneath the Balcones Fault zone (BFZ). The Moho may be undetectable due to serpentinization of the upper mantle from water leaking through the BFZ, a possibility supported by the existence of a low velocity anomaly in the upper mantle beneath the BFZ found by surface wave modeling. Ps and Sp RFs also found evidence for a low velocity zone in the 50-70 km depth interval near the shoreline of the GCP. Ps RF imaging suggests this LVZ may be a piece of subducted lower crust emplaced during the Ouachita orogeny with a trapped mantle wedge above it. These observations are supported by gravity modeling.